Fixing process for photoelectrophoretic imaging

ABSTRACT

METHODS AND APPARATUS FOR PHOTOELECTROPHORETIC IMAGING UTILIZING THERMO-ADHESIVE LAYERS FOR IMAGE TRANSFER AND FIXING.

Dec. 12, 1972 v, 5, MEIHAJLQV ETAL 3,705,797

FIXING PROCESS FOR PHOTOELECTROPHORETIC IMAGING Original Filed March 20,1970 2 Sheets-Sheet 1 FIG 9 I7 I IIITHII mum I I'I' ""16 r J\ INVENTORSvsevoLoo s. MIHAJLOV BY LEONARDM.CARREIRA ATTORNEY Dec. 12, 1972MIHAJLOV ETAL v 3,705,797

FIXING PROCESS FOR PHOTOELECTROPHORETIC IMAGING Original Filed March 20,1970 2 Sheets-Sheet 2 lllllll'l '"l'l'Il'lllvI United States Patent O3,705,797 FIXING PROCESS FOR PHOTOELECTROPHORETIC IMAGING Vsevolod S.Mihajlov and Leonard M. Carreira, Penfield, N .Y., assignors to XeroxCorporation, Rochester, NY. Continuation of application Ser. No.808,921, Mar. 20, 1969, which is a continuation-in-part of applicationsSer. No. 459,860, May 28, 1965, Ser. No. 677,706 and Ser. No. 677,707,both Oct. 24, 1967, all now abandoned. This application Feb. 18, 1970,Ser. No.

Int. Cl. G03g 13/22 U.S. Cl. 96-1 R 9 Claims ABSTRACT OF THE DISCLOSUREBACKGROUND OF THE INVENTION This invention relates in general to imagingsystems and more specifically, to an improved electrophoretic imagingsystem. This application is a continuation of application Ser. No.808,921 filed Mar. 20, 1969. Ser. No. 808,921 was a continuation ofapplication Ser. Nos. 459,860 filed May 28, 1965, Ser. No. 677,706 and677,707 filed Oct. 24, 1967, all of the above now abandoned.

There has been recently developed an electrophoretic imaging systemcapable of producing color images which utilizes electricallyphotosensitive particles. This process is described in detail andclaimed in copending applications Ser. Nos. 384,737 now U.S. Pat.3,384,565, issued May 21, 1968; 384,681 now U.S. Pat. 3,384,566, issuedMay 21, 1968 and 384,680 now U.S. Pat. 3,383,993, issued May 21, 1968,all filed July 23, 1964. In such an imaging system, variously coloredlight-absorbing particles are suspended in a nonconductive liquidcarrier. The suspension is placed between electrodes, subjected to apotential difference and exposed to an image. As these steps arecompleted, selective particle migration takes place in imageconfiguration, providing a visible image at one or both of theelectrodes. An essential component of the system is the suspendedparticles which must be electrically photosensitive and which apparentlyundergo a net change in charge polarity upon exposure to activatingelectromagnetic radiation, through interaction with one of theelectrodes. In a monochromatic system, particles of a single color maybe used, producing a single colored image equivalent to conventionalblack-and-white photography. In a polychromatic system, the images areproduced in natural color because mixtures of particles of two or moredifferent colors which are each sensitive only to light of a specificwave-length or narrow range of wave-lengths are used. Particles used inthis system must have both intense and pure colors and be highlyphotosensitive.

After the exposure and particle migration steps are completed, theelectrodes are separated and the carrier liquid is allowed to evaporate.This leaves images on one or both of the electrodes made up ofselectively deposited particles. The carrier liquid may contain a smallproportion of a wax or other binder which would serve to bind theparticles together in the images. However, if more than a very smallamount of binder material is used, undesirable interference with theimaging process takes place. Thus, the images are at this time in afragile and easily damaged condition. It has been suggested that atransparent sheet be laminated over the images, or a transparent binderresin be sprayed over the images to form a protective coating. While,when carefully done, these techniques will protect the image, the imageis often damaged during the application of the protective mate-3,705,797 Patented Dec. 12, 1972 rial. These protective techniques arenot suitable for a mechanized system. Also, when it is desired totransfer the image from the electrode material to a receiving sheet, thedangers of smudging or otherwise damaging an unfixed image is verygreat. Thus, there is a continuing need for a better system for fixingthe particulate image formed on the electrode surface and/or forpermitting transfer of said image to a receiving sheet.

SUMMARY OF THE INVENTION It is, therefore, an object of this inventionto provide a method of fixing a particulate electrophoretic image whichovercomes the above-noted disadvantages.

It is another object of this invention to provide a method of protectingan electrophoretic image from damage.

It is another object of this invention to provide a method oftransferring an electrophoretic image to a receiving sheet.

It is still another object of this invention to provide anelectrophoretic imaging method capable of producing images which may behandled without damage thereto.

It is still another object of this invention to provide a continuousmethod of forming electrophoretic images and protecting them againstabrasion damage.

The foregoing objects and others are accomplished in accordance withthis invention by providing a thermoadhesive layer, which when broughtinto contact with an electrophoretic particulate image in a softenedstate, will permit the particles to be embedded in the layer and bepermanently held by the layer when it is permitted to reharden. Thisthermo-adhesive layer may be coated on the electrode upon which an imageis to be formed. After the image is formed, the thermo-adhesive layer issoftened and the image is pressed against the layer, thereby em beddingthe particles therein. When the thermo-adhesive is permitted toreharden, the particles are permanently set therein and are therebyprotected against damage from contact with other objects. Alternatively,the thermoadhesive layer may be coated on a receiving sheet. After anelectrophoretic image is formed on an electrode, the receiving sheet isheated to soften the thermo-adhesive and brought into contact with theimage. The adhesive picks up the particulate image and when rehardenedprovides a protective image matrix.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages of this improvedelectrophoretic imaging process and apparatus will become apparent uponconsideration of the following detailed disclosure of the invention;especially when taken in conjunction with the accompanying drawingswherein:

FIG. 1 shows a side view of a simple exemplary system for carrying outthe process of this invention wherein a thermo-adhesive image fixinglayer is coated on the imaging electrode; I

FIG. 2 shows a simple exemplary receiving sheet capable of acceptingtransfer of an electrophoretic particulate image from an imagingelectrode; and,

FIG. 3 shows a continuous system for forming, transferring and fixingelectrophoretic particulate images.

Referring now to FIG. 1, there is seen a transparent electrode generallydesignated 1 which, in this exemplary instance, is made up of a layer ofoptically transparent glass 2 overcoated with a thin opticallytransparent layer 3 of tin oxide, commercially available under the nameNESA glass. This electrode will hereafter be referred to as theinjecting electrode. On the surface of injecting electrode 1 is a layerof a thermo-adhesive material 13, coated on, for example, a conductivecellophane film; layer 13 is capable of becoming tacky when heated byinfrared lamp schematically shown at 14.

Over the solidified thermo-adhesive layer 13 is coated a thin layer 4 offinely divided photosensitive particles dispersed in an insulatingliquid carrer. The term photosenstive, for the purposes of thisapplication, refers to the properties of a particle which, onceattracted to the injecting electrode, will migrate away from it underthe influence of an applied electric field when it is exposed toactivating electromagnetic radiation. For a detailed theoreticalexplanation of the apparent mechanism of operation of the imagingprocess, see the above-mentioned copending applications Serial Nos.384,737 now US. Pat. 3,384,565; 384,681 now US. Pat. 3,384,566; and384,680 now U.S. Pat. 3,383,993, the disclosures of which areincorporated herein by reference. Adjacent to the liquid suspension 4 isa second electrode 5, hereinafter called the blocking electrode, whichis connected to one side of the potential source 6 through a switch 7.The opposite side of potential source 6 is connected to the injectingelectrode 1 so that when switch 7 is closed, an electric field isapplied across the liquid suspension 4 and thermoadhesive layer 13between electrodes 1 and 5. An image projector made up of a light source8, a transparency 9, and a lens 10 is provided to expose the dispersion4 to a light image of the original transparency 9 to be reproduced.Electrode is made in the form of a roller having a conductive centralcore 11 connected to the potential source 6. The core is covered with alayer of a blocking electrode material 12, which may be, for example,baryta paper. The pigment suspension is exposed to the image to bereproduced while a potential is ap plied across the blocking andinjecting electrodes by closiug switch 7. Roller 5 is caused to rollacross the top surface of injecting electrode 1 with switch 7 closedduring the period of image exposure. This light exposure causes exposedpigment particles originally attracted to electrode 1 to migrate throughthe liquid and adhere to the surface of the blocking electrode, leavingbehind a pigment image on the surface of the thermo-adhesive layer 13which is a duplicate of the original transparency 9. Particles adheringto the surface of a blocking electrode 5 may be cleaned therefrom andthe exposure steps repeated, if desired. The additional steps ofexposing and cleaning the blocking electrode have been found to increasecolor purity and color balance. After exposure, the relatively volatilecarrier liquid evaporates off, leaving behind the pigment image. At thistime, the pigment pigment image is very susceptible to damage by contactwith any object since it consists of, in effect, loosely held particles.The image may be fixed or set by heating the thermo-aclhesive layer 13,as by infrared lamp 14. Heating the thermo-adhesive layer causes it tobecome tacky as further explained below and causes the image particlesto be embedded therein. The thermo-adhesive layer is allowed to hardenand may then be stripped from the injecting electrode and handledwithout fear of smudging or other damage from surface contact.

In an alternative process, the pigment suspension 4 may be coateddirectly on the NESA glass surface 13, omitting the intermediatethermo-adhesive layer 13. After the imaging steps are carried out asdiscussed above, a particulate image remains on the NESA glass surface.The particulate image may then be transferred to a receiving sheet suchas is shown in FIG. 2. This receiving sheet comprises a base layer 15which may be paper, cellophane, or other suitable materials. On thisbase layer is coated a layer of a thermo-adhesive material 16, the sameas layer 13 discussed above. This layer may be made tacky by heating bycontact, or by infrared radiation, and be brought into contact with theparticulate image on the electrode. 1. Or, the solidified thermoadhesivelayer may be brough into contact with the particulate image and thenheated. In either case, the particles become embedded in thethermo-adhesive layer and are tired t erein hen the l y r s allcwsd t9reharden. Th

receiving sheet is then removed from the injecting electrode and may behandled in any conventional manner.

FIG. 3 shows an exemplary system for continuously forming aphotoelectrophoretic image, transferring the image to a receiving sheetand fixing the image thereon. In the embodiment of this figure, thetransparent electrode 1 and the blocking electrodes are the same as inthe above-discussed embodiment of FIG. 1. Here, however, a tractor 18 iscoupled to the blocking electrode 5 to automatically transfer and fixthe positive image formed on the NESA glass surface 3. The tractor 18comprises a frame 19 which supports the blocking electrode 5 and imagetransfer means for movement across the imaging surface. The transfermeans consists of a continuous web 20 of transfer material, e.g. paper,on which is coated a thermo-adhesive layer. The web is mounted on supplyroller 21 and is adapted to pass in contact with heating guide roller 22and cooling guide roller 23 on its way to take-up roller 24. Heatingguide roller 22 is provided with internal heating means capable ofheating the thermo-adhesive layer above its softening temperature as itpasses said roller. This heating means, schematically shown at 25 maycomprise any conventional means, e.g. a pipe admitting steam to theinterior of the roller, electrical resistance heating means connected toa power supply or Peltier junction heating means connected to a powersupply. Cooling guide roller 23 is supplied with coolant sufiicient tocool the thermo-adhesive below its softening point as it passes saidroller. The cooling means, schematically shown at 26, may comprise anyconventional means, e.g. cooling water piped to the roller, or Peltierjunction means connected to a power supply. In operation, a tri-mix 4 isapplied to the transparent electrode 1. The tri-mix is exposed to animage and the tractor and blocking electrode are moved from left toright across the imaging surface. As the blocking electrode passes theimaging surface, unwanted particles migrate to the blocking electrodesurface leaving a positive particulate image on the NESA .glass surface3. As the tractor reaches the NESA surface, web 20 contacts the NESAsurface 3 without relative movement with respect thereto. Thethermo-adhesive surface on web 20 is softened by heat applied at roller22. The particulate image becomes embedded in the softened surface ofthe thermoadhesive layer. Then as the thermo-adhesive passes coolingroller 23, it is rehardened and wound up on takeup roll 24'. When thetractor and blocking electrode reach the end of their travel, brush 27cleans unwanted pigment from the surface of blocking electrode 5. Thetractor is then raised slightly and returned to the starting positionwithout again contacting the transparent electrode surface. The dashedline 28 schematically indicates the path taken by axle 29 of the rollerelectrode during the imaging and return movements. As can be seen, thedevice is capable of continuously forming, transferring, fixing andstoring photoelectrophoretic images.

The thermo-adhesive layers 13 and 16 as described above may comprise anysuitable materials. The only requirements are that they be solid at roomtemperatures and be capable of softening and becoming tacky at reasonably elevated temperatures. When the layer is to be coated directlyon the injecting electrode surface, there is the further requirementthat they have proper conductive characteristics. Where thethermo-adhesive layer is coated on the blocking electrode surface thelayer should have a resistivity between 10 and 10 ohm centimeters. Thethermoadhesive layer preferably comprises a binder material and athermo-solvent for the binder. The thermo-solvent comprises a materialthat is solid at room temperature and melts slightly above roomtemperature, thereby causing the binder-solvent layer to be tacky andpermit particles in contact therewith to be embedded therein. Where theli e! imag i t9 be viewed by projection, the thermo-adhesive layershould be transparent.

Any suitable mixture of binder resin and thermosolvent may be used inthe process of this invention. Optimum results have been obtained withmixtures of Vinylite VYNS, a vinyl chloride-vinyl acetate copolymer,available from Union Carbide Corporation, and Santolite MHP, an arylsulfonamide-formaldehyde copolymer available from Monsanto;polyvinylpyrrolidone-vinylacetate copolymer and Santolite MHP, andVinylite VYHO and Aroclor 4465, a blend of chlorinated biphenyls andchlorinated triphenyls. Best results have been obtained with, andtherefore, the preferred-formulation is, a mixture of 1 part by weightEXON-470, a vinylchloride-vinylacetate copolymer available fromFirestone, about 1 part by weight Santicizer 1-H, a sulfonimide resinavailable from Monsanto, dissolved in about parts acetone. Thisformulation is preferably coated to a thickness of about 0.5 mil anddried. Any other suitable mixture of binder resin and thermosolvent maybe used. Typical binder resin materials include polyethylenes,polystyrenes, copolymers of vinylchloride and vinylacetate, copolymersof 'vinylpyrrolidione and vinylacetate, polyvinyl methacrylates,polyvinyl propylene, polyvinylchloride, cellulose acetate, chlorinatedrubber, and mixtures and copolymers thereof. Typical thermosolventshaving melting points slightly above room temperature include (withmelting temperatures in parentheses) triphenyl phosphate (48 C.);dicyclohexyl phthalate (63 C.); diphenyl phthalate (69 C.); Aroclor 5442(4652 C.) a chlorinated polyphenyl available from Monsanto; Santicizer 3(58 C.), N-ethyl-p-toluenesulfonamide, available from Monsanto;Santolite MHP (62 C.) a sulfonamideformaldehyde resin available fromMonsanto; Santicizer 1-H (82 C.) N-cyclohexyl-ptoluenesulfonarnide,available from Monsanto; acenaphthene (94 C.) acetanilide (113 C.);o-acetoacetotoluidide (105 0.); o-acetotoluidide (101 C.);o-chloroacetoacetanilide (103 C.); 2-chloro-4-nitroani1ine (106 C.); pdibromobenzene (87 (1.); p,p-methylenedianiline (93 C.); alpha-naphthol(95 C.) beta-naphthol (85 (3.); Z-naphthylamine (110 C.), m-nitroaniline(112 C.); 4-nitrobiphenyl (97 C.); sorbitol hexoacetate (98 C.);2,4-toluenediamine (97 C.) and mixtures thereof.

Any suitable ratio of binder to thermo-solvent may be used. Ratios offrom 0.5 to 4 parts binder resin for each part of thermo-solvent may beused. For optimum results, a ratio of binder resin to thermo-solvent ofabout 1:1 is preferred. This ratio may vary depending upon theparticular binder resin and thermo-solvent selected. The thickness ofthe thermo-adhesi-ve layer should preferably be between 0.1 and 4 mils.The optimum balance between effective transfer and economy of materialshas been found to occur with a thermo-adhesive thickness of about 0.5mil.

DESCRIPTION OF PREFERRED EMBODIMENTS The following examples furtherspecifically define the present invention with respect to the use ofthermoadhesive layers to fix and/or transfer electrophoretic images. Theparts and percentages are by weight unless otherwise indicated. Theexamples below are intended to illustrate various preferred embodimentsof the electrophoretic image fixing and transferring process of thisinvention.

All of the following Examples I-IV are carried out in an apparatus ofthe general type illustrated in FIG. 1. In different examples, however,the thermo-adhesive layer may be placed on the NESA glass substrate, oron the blocking electrode surface, or on a separate receiving sheet suchas shown in FIG. 2. The NESA glass surface is connected in series with aswitch, a potential source, and a conductive center of a roller having acoating of baryta paper on its surface. The roller is approximately 2%inches in diameter and is moved across the plate surface at about 1.45centimeters per second. The plate employed is roughly 3 inches squareand is exposed with a light intensity. of 8,000foot candles as measuredon the uncoated NESA glass surface. Unless otherwise indicated, 7percent by weight of the indicated pigments in each example aresuspended in Sohio Odorless Solvent 3440, a kerosene fraction availablefrom Standard Oil of Ohio, and the magnitude of the applied potential is2,500 volts. Exposure is made with a 3,200 K, lamp through a multicolorKodachrome transparency.

EXAMPLE I Equal parts by weight of Vinylite V YNS, a vinylchloridevinylacetate copolymer available from Union Carbide Corporation andSantolite MHP, an aryl 'sulfonirnide formaldehyde copolymer availablefrom Monsanto are mixed in an acetone solvent and coated to a thicknessof about 10 microns on a cellophane film which is placed on the NESAglass substrate. A tri-mix comprising a cyan pigment, Monolite Fast BlueGS, the alpha form of metal free phthalocyanine, C.I. No. 74100,available from Arnold Hoffman Co.; a magenta pigment, Vulcan 'Fast RedBBE toner 35-2201, 3,3'-dimethoXy-4,4'-biphenylbis (1"phenyl-3"-methyl-4-azo-2"-pyrazolin-5"-one), 0.1. No. 21200, availablefrom Collway Colors Company; and a yellow pigment, Indofast YellowToner, flavanthrone, 0.1. No. 70600, available from Harmon ColorsCompany, is dispersed in about parts Sohio Odorless Solvent 3440, andcoated onto the thermo-adhesive coated cellophane layer on the NESAglass surface. A potential is imposed across the suspension, and animage is formed as discussed above. After the image is formed, residualcarrier is allowed to evaporate and the coated cellophane surface isheated until the thermo-adhesive softens with an infrared lamp. A rollerhaving a fluorocarbon coated surface is rolled across theth'ermo-adhesive layer to press the particulate image into thethermo-adhesive surface. The heating is then stopped and thethermo-adhesive surface is allowed to reharden. An excellent imageresults, with a tough surface resistant to abrasion damage.

EXAMPLE 11 Equal portions of a vinyl pyrrolidone-vinylacetate copolymeris mixed with Santolite MH'P in an acetone solvent condition. Thesolution is coated onto a baryta paper blocking electrode surface to athickness of about 10 microns and allowed to harden thereon. A tri-mixis prepared, comprising a cyan pigment, Cyan Blue GTNF, the beta form ofcopper phthalocyanine, C.I. No. 74160, available from Collway ColorsCompany; a magenta pigment, Quindo Magenta RV-6803, a quinacridonepigment available from Harmon Color Co.; and a yellow pigment AlgolYellow GC, 1,2,5,'6-di(C,C-diphenyl)- thiazole-anthraquinone, C.I. No.67300, available from General Dyestuffs Co., in about 100 parts SohioOdorless Solvent 3440. This tri-mix is coated onto the NESA glasssubstrate and an image is produced as in Example I. The blockingelectrode is cleaned of residual unwanted pigments, then the surface isheated until it becomes tacky. Then the roller is rolled across the NESAsurface picking up and embedding the image particles therefrom. Thethermo-adhesive is allowed to re-harden. 'Ihe baryta paper carrying thepigment containing adhesive layer is removed from the roller electrodeand examined. An image of excellent quality with a tough, abrasionresistant surface is seen.

EXAMPLE III Equal proportions of Vinylite VYNS and Aroclor 4465, a blendof chlorinated bi-phenyls and chlorinated triphenyls, available fromMonsanto, is dissolved in acetone and the solution is coated onto apaper sheet to a thickness of about 5 microns. A tri-mix is prepared asin Example 11 above and is coated on the NESA glass substrate. An imageis produced thereon as in Example I above. The thermo-adhesive coatedsheet is heated by placing it on a platen held at about 90 C. When thethermo-adhesive has reached a tacky state, it is removed from the platenand pressed down on the particulate image on the NESA sheet. Uponremoval of the sheet and rehardening of the thermo-adhesive, anexcellent image is seen with a hard abrasion resistant surface.

EXAMPLE IV Equal proportions of EXON 470, and Santicize r l-H aredissolved in acetone and the soltuion is coated onto a long paper weband allowed to dry. The web is wound on a roller and placed in a devicesuch as shown in FIG. 3. A tri-mix prepared as in Example I is coatedonto the NESA glass electrode. An image is produced as in Example I.Unwanted pigment particles migrate to the blocking electrode and areremoved therewith, leaving a particulate image remaining on the NESAsurface. As the tractor passes over the NESA plate, the particulateimage is pressed into the heated thermo-adhesive surface. As the paperweb is wound past the cooled roller, the thermoadhesive hardens,resulting in a tough image surface which can be wound onto the takeuproller. The imaging operations may then be repeated without delay.

Although specific components and proportions have been stated in theabove description of preferred embodiments of the thermo-adhesive layer,other suitable materials, as listed above, may be used with similarresults. In addition, other materials may be added to the mixture tosynergize, enhance, or otherwise modify its properties. For example,whiteners may be added to the thermoadhe'sive to brighten the image,especially where an inex-v pensive grade of paper is used for transfer.Where the thermo-adhesive layer is used on the injecting electrode,additives may be included to increase the conductivity of the layer asdesired.

Other modifications and ramifications of the present invention willoccur to those skilled in the art upon a reading of the disclosure.These are intended to be included within the scope of this invention.

What is claimed is:

1. The method of photoelectrophoretic imaging comprising the steps of:

(a) forming a layer of an imaging suspension comprising electricallyphotosensitive particles in a substantially insulating carrier liquid ona transparent conductive electrode;

(b) providing a solid layer comprising a resinous binder and athermo-solvent for said resinous binder on a second electrode; saidsolid layer having a resistance of between and 10 ohm centimeters;

(c) exposing said suspension to a pattern of electromagnetic radiationwhile contacting the' free surface of said imaging suspension with saidsecond electrode and applying a potential difference between said secondelectrode and said transparent conductive electrode until an image isformed on said second electrode;

(d) heating said solid layer until said layer is at least partiallysoftened; and,

(e) cooling said layer to fix said image.

2. The methodof claim 1 wherein said resinous binder thermo-solventlayer comprises from about 0.5 to about 4 parts resin based on one partthermo-solvent.

3. The method of claim 1 wherein said resinous binder thermo-solventlayer has a thickness of from about 0.1 mil toabout 4 mils.

4. The method of claim 1 wherein said resinous binder is selected fromthe group consisting of a vinylchloridevinylacetate copolymer and avinylpyrrolidone-vinylacetate copolymer.

5. The method of claim 1 wherein said thermo-solvent is selected fromthe group consisting of an aryl sulfonamide-formaldehyde copolymer and-a blend of chlorinated biphenyls and chlorinated triphenyls.

6. The method of photoelectrophoretic imaging comprising the steps of:

(a) providing a solid layer comprising a resinous binder and athe-rmo-solvent for said resinous binder on a transparent conductiveelectrode, said layer having a thickness of from about 0.1 mil toabout-4 mils;

(b) forming a layer of an imaging suspension comprising electricallyphotosensitive particles in a substantially insulating carrier liquid onsaid solid layer;

(c) exposing said imaging suspension to a pattern of electromagneticradiation while contacting the free surface of said imaging suspensionwith a second electrode and applying a potential difference between saidsecond electrode and said transparent conductive electrode until animage is formed on said solid layer;

(d) heating said solid layer until said solid layer is at leastpartially softened; and,

(e) cooling said solid layer to fix said image.

7. The method of claim 6 wherein said resinous binder thermo-solventlayer comprises from about 0.5 to about 4 parts resin based on one partthermo-solvent.

8. The method of claim 6 wherein said resinous binder is selected fromthe group consisting of a vinylchloridevinylacetate copolymer, and avinylpyrrolidone-vinylacetate copolymer.

9. The method of claim 6 wherein said thermo-solvent is selected fromthe group consisting of an aryl sulfonamide-formaldehyde copolymer and ablend of chlorinated' biphenyls and chlorinated triphenyls.

References Cited UNITED STATES PATENTS

